It's most likely listed as AWK in the flight Plan (Air Work) and cleared to operate in a defined area and level(s) specifically. . Generally dropping anything from a Plane is Prohibited, but there would be exceptions. Banners are draggy but not dense. The load, (drag) is generally enough to require all the reserve power the Motor can make continuously. (Max visual impact for money) Often with a finer pitch prop fitted. I've rented Banner towing Planes and they are usually hacks you wouldn't fly (in normal circumstances). Being a (very) cheap hire rate was my reason. That's when I needed HOURS in my log book. Nev